Synchronous rectifiers (SRs) are important for high-output-current LLC converters in order to reduce the secondary conduction loss. Conventional SR driving schemes are not suitable for high-frequency LLC converters: the analog driving schemes will lose a large portion of duty cycle due to SR parasitic inductance; and the digital driving schemes will require a much higher cost digital controllers in high-frequency operation. In this paper, a digital implementation of adaptive SR driving scheme for high-frequency LLC converters with cost-effective microcontroller is proposed. The proposed SR driving scheme senses SR drain-source voltage in order to detect body diode conduction, and tune the SR on-time every several switching cycles using ripple (asynchronous) counter to minimize body diode conduction. The proposed digital implementation has a simple control scheme, requires only low-cost digital controllers and minimizes controller resources utilization. More importantly, to cooperate with the closed-loop control, adaptive SR driving and closed-loop control are executed synchronously and SR on-time is modified accordingly during fast transient response to guarantee safe operation. With these techniques, the proposed adaptive SR driving can be embedded into the digital controllers with little extra cost. Experimental results are demonstrated on a 500-kHz 1-kW 400 V/12 V LLC converter with a 60-MHz microcontroller and a ripple counter.
